Combustible cartridge case of felted fibrous material with synthetic resin and process



United States Patent Ofiice 3,280,745 Patented Oct. 25, 1966 COMBUSTIBLE CARTRIDGE CASE OF FELTED FIBROUS MATERIAL WITH SYNTHETIC RESIN AND PROCESS George E. Brown, Fairfax County, Va., assignor to Atlantic Research Corporation, Fairfax County, Va., a corporation of Virginia No Drawing. Filed Apr. 26, 1965, Ser. No. 451,075

14 Claims. (Cl. 102-43) This invention relates to a new and improved combustible, shaped, felted, and fibrous nitrocellulose product and to the method for its preparation. More specifically, it relates to an improved combustible casing material for propellant and explosive powders and grains. Still more particularly, it pertains to a combustible cartridge casing material having superior physical properties.

Consumable cartridge casings made of felted cellulose fibers admixed with sufiicient nitrocellulose fibers to make the casings combustible are known. Such casings are prepared by dispersing the fibers in a liquid in the form of a thick slurry and in the presence of a binder, preferably a synthetic organic resin, depositing the slurry on a casing form, removing the bulk of the liquid, compacting the fibers, and heating the deposited casing to dry it and to cure the resin binder. The resultant casing product is a fairly rugged container which exhibits many advantages over comparable metal casings. See for example, Ordnance, vol. XLVII, No. 254, September- October 1962, pages 23 1 234.

One of the serious problems encountered in preparation of a combustible casing suitable for munition type applications is that of achieving high tensile strength in the casing without significantly impairing the high porosity needed in the casing wall to obtain efficient combustion of the casing material. This invention provides an effective means for solving the aforegoing problem.

Broadly speaking, the invention comprises subjecting a cured casing of the type described above to a solvating treatment with solvent for nitrocellulose for a period of time sufilcient to permit penetration of the solvent into the pores of the casing, retaining the solvent in the pores until at least outer surfaces of the nitrocellulose fibers of the casing are solubilized, or tackified, and then removing the solvent.

The treated casing is a product having highly increased tensile strength, load bearing capacity and rigidity. This casing is able to withstand handling and shock with far less danger of damage than is experienced with untreated casings.

The advantageous increase in physical properties of the product of the invention depends in part on the type of synthetic organic resin binder which is present in the casing; in part on the solvent which is used to treat the casing; in part on the method of contacting of the casing with'the solvent, e.g., by dipping, spraying, or vapor treatment; in part on the temperature of the treating process; and in part on the time duration of contact during treatment.

Binders of the synthetic organic resin type are commonly used in the manufacture of felted cellulose-nitrocellulose products from fibers. Not all resins are effectively suited for this purpose, since the binder resin must be compatible with the nitrocellulose and must not interfere with the combustion of the product, especially if the product is intended for use as a combustible cartridge casing. For the practice of the present invention, the binder present in the untreated product also should be one which does not seriously impair the solvation of the nitrocellulose fibers by the solvent being used.

A suitable binder material is, for example, one derived from a water-dispersed thermosetting adhesive resin composition comprising a self-reactive, aqueous emulsion copolymer containing at least by weight, of vinyl acetate together with at least one other polymerizable comonomer containing at least one functional group selected from the group consisting of the glycidyl, methylol, including alkyl methylol, ureido, including alkylene ureido, hydroxyl, amine, carboxyl and alkoxycarbonyl groups. Binders of this type are disclosed and claimed in the copending application of DeFries and Godfrey, Serial No. 450,215, filed April 22, 1965. An example of such binder is copolymer of vinyl acetate in combination with methyl methacrylate in the ratio of 9624 parts by weight.

It should be understood that this invention is not directed to a novel binder material. However, the binder which is already present in the untreated product must be duly considered, since the binder constitutes from about 15 to 35% of the cured casing material. The binder can be soluble in the solvent used. But it must neither be so soluble that it is extracted by the solvent, nor so insoluble that it prevents the solvent from solvating the nitrocellulose fibers held together by the binder. In the event that the solvent were to extract much of the binder, the form as well as the combustible characteristics of the product could be adversely altered. Also, were the binder material removed and the cellulose fi-bers held together solely by the solvated, dried nitrocellulose fibers, the resulting product would be brittle and substantially non-porous, and consequently impaired for use as a combustible cartridge casing. Accordingly, the invention is best practiced with a casing material made with a synthetic organic resin binder which preferably is only moderately soluble in the solvent system used.

The solvent used to practice the invention must be a solvent or solvent system which dissolves or solvates nitrocellulose fibers in the presence of the particular binder used to bind the untreated product. Nitrocellulose is known to be soluble in a variety of true solvents, chief among which are esters such as ethyl, butyl, and amyl acetates; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; specific alcohols, such as methanol; and amides, such as ethyl amide. Nitrocellulose is also known to be solvated by solvent materials which are used as diluents with the above true solvents. Examples of the diluent materials are specific alcohols, e.g., ethanol, butanols; aliphatic hydrocarbons, e.g., hexane, heptane, octane; and aromatic hydrocarbons, e.g., toluene, xylenes. The solvent used, accordingly, can be any of the true solvents or mixtures thereof, e.g., acetone with methanol, and any mixture of a true solvent and 21 diluent, e.g., methanol with n-butanol. Acetone is preferably used as the solvent.

The amount of solvent used is not critical except that suificient solvent should be used to cause adequate solvation of the nitrocellulose fibers in the casing so that upon removal of the solvent the nitrocellulose and cellulose fibers bond with each other. The treated casing is left in a significantly strengthened condition insofar as physical properties are concerned, and its porosity is not destroyed or detrimentally altered.

The method used in treating the product with solvent will depend somewhat on the form of the product. The product can be in the form of fiat sheet, tape of suitable thinness for rolling, hollow forms, tubing, foraminous all surfaces subjected to a solvent spray, followed by the drying step after allowing an appropriate time for solva- 3 tion to take place. Also, the product can be exposed to solvent vapor in an enclosed vapor chamber. Other methods will be apparent to those skilled in the coating and impregnating arts.

From Examples 2 and 3 it is seen that the acetone dip treatment increases ultimate stress, i.e., tensile strength, over 100% over that of the untreated casing of Example 1 despite the greater thickness of the latter casing. The

The temperature of carrying out the process is not stiffness of the treated casing is more than doubled, as critical. However, temperatures higher than ambient shown by the Youngs modulus data. The load bearing preferably are used in order to accelerate solvation and capacities are increased by 54-60%. subsequent removal of the solvent. Example 4 shows that methanol is a good solvent for The time duration of contact must be controlled to practice of the process but not as good as acetone. provide adequate time for solvation to occur. However, Examples 5 and 6 show that n-butanol and isopropanol prolonged contact of the nitrocellulose with solvent, parare not as advantageously effective as methanol. ticularly at an elevated temperature, can result in extrac- Examples 7-9 show that the vapor method of treatment tion of nitrocellulose or binder or both into the solvent is not as efficient as the dip method, timeWise. and can cause degradation of the product both as to Example 10 shows that the spray method also is less strength and as to its combustible properties. efiicient than the dip method in improving resistance to In view of the highly combustible character of the stress and to load bearing capacity. product, dur precautions must be taken to avoid the The treated casing materials of Examples 2-10 burned hazards of fire and explosions present during all phases rapidly and cleanly on ignition. of the processing. Usual practices followed by those skilled in the art should be strictly adhered to. Examp 11 The advantages resulting from the practice of the invention are shown in the table where the test results of A ,senes of abollt 0'160 Wa11 thlcknFs combustlble Example 1, obtained with a combustible cartridge casing c'flrtndge cases havmg nommal composltlon of 40 Parts which was not treated by the process of the invention, nitrocellulose fibers'(105 13'5% fibers are compared with test results of Examples 240 49 parts of the same synthetic orgarnc res1n bmder as tained with solvent treated cartridge casings. used, Examples 1401 1 Parts of dlphenylamme and a The cartridge casings of Examples were made by negl1g1ble amount of residual water were tested before the Same process and each had the following nominal and after treatment for 20 seconds wlth acetone by the composition. d1pp1ng, or 1mmers1on, method. The following average results were obtained on samples cut from the walls, Component: Weight, percent which were about 0.133" in thickness after the treatment:

Nitrocellulose (10.513.5% N) 57 Kraft fiber 14 Resin binder solids (copolymer of vinyl D$$2ms;":::::::::::::::::::::::::" Sigh-tit Resldual water 4 Before 132 1,986 129,800 I After 151 2, 227 153,600

Casmgs 1n Examples 2-6 were dipped into the solvent Percent Increase 11.8 at room temperature for about 20 seconds and allowed to dry at room temperature.

Casings in Examples 7-9 were exposed to acetone vapors at the boiling temperature of acetone at atmospheric Burning rate Was Slower than that in EXamPIeS pressure for varying periods of time, i.e., from 60 to 240 FY9111 Example 11 it is to be noted that the ratio seconds. of binder to fibers is 1:1 and that, although initial values The casing in Example 10 was sprayed with acetone are relatively high Compared to those of casings 0f with a pressure-fed paint spray gun with the tank presamples increases in load bearing p y, ultimate sure set at 18 to 20 p.s.i. with 15 to 20 p.s.i. pressure at Stress, h rigidity are relatively The reason for the nozzle The casing was sprayed and dried at room the low increases is believed due to the presence of the temperature h gh amount of b1nder, which makes solvation of the The wall thickness of each casing was substantially the 111th ocelhllose' more dlfiicult than that Occurring in the same as that of Examples 1, i.e., about 0.160". During cas lng s used Examples Where the binder t0 fibers treatment, some shrinkage occurs. The acetone dipped who 15 casings of Examples 2 and 3 underwent a wall thickness Example 12 shrinkage of about 0.037 inch. This is not considered significant from the standpoint of operation of the proc- Wall samples from a series of about 0.160 thick comess. However, where dimensions are critical, the shrinkustible cartridge casings having a nominal composition age must be taken into account in preparing dies and of 75 parts nitrocellulose fibers (10.5-13.5 N), 20 parts molds. The sprayed casings underwent a wall thickness kraft fibers, 3 parts diphenylamine and 2 parts water shrinkage of about 0.01 inch. (no synthetic organic resin binder being present) were TABLE Test Data at 73 F. Percent Increase Over Ex. No. 1 Ex. Wall Time of Type of No. Thickness Solvent Exposure, Exposure seconds Max. Ultimate Youngs Max. Ultimate Young's Load, lbs. Stress, Modulus Load, lbs. Stress Modulus p.s.l.

.161 None 54.7 676 43, 950 .123 Acetone 84.5 1,371 98,650 54.3 101.8 102.5 .125 do. 87.5 1,405 118,500 60.0 108.0 170.0 .141 Methanol 70.5 998 99,500 29.0 47.5 102.9 .159 n-Butanol-.. 56.0 709 62,600 2.3 4.8 43.0 .165 IsopropanoL 68.2 701 60,000 6.3 3.8 36.8 .147 Acetone- 69.5 943 173,000 27.0 39.5 293.0 65.8 851 64,100 20. 1 26. 0 41. 5 14 59. 3 797 64, 500 18. 5 18. 0 47. 0

tested before and after treatment for 20 seconds by dipping in acetone. The wall thickness averaged 0.09" after treatment. The following results were obtained:

treated, as can be seen from the decrease in load bearing capacity of the treated material. Thus, while the tensile strength and stiffness of the treated product were improved by the acetone treatment, the product was made brittle. Porosity of the product was reduced, as evidenced by the wall thickness shrinkage experienced, in comparison to samples of treated material bound as in Examples 1-11.

While the invention has been described in its preferred embodiments it is to be understood that it may be practiced in other embodiments within the scope of the claims as will be clear to those skilled in the art.

I claim:

1. The process which comprises subjecting a combustible, shaped, felted, and fibrous material consisting essentially of nitrocellulose fibers, cellulose fibers and synthetic organic resin binder therefor to a solvating treatment with solvent for nitrocellulose for a period of time suflicient to solubilize at least the outer surfaces of said nitrocellulose fibers, and then removing the solvent.

2. The process according to claim 1 in which the solvent is a true solvent for nitrocellulose.

3. The process according to claim 2 in which the solvent is a ketone.

4. The process according to claim 3 in which the solvent is acetone.

5..The process according to claim 2 in which the solvent is methanol.

6. The process according to claim 2 in which the solvent is an ester.

7. The process according to claim 1 in which the solvent is a mixture of a true solvent and a diluent solvent.

8. The process according to claim 7 in which the diluent solvent is a butanol.

9. The process according to claim 1 in which the material is in the shape of a combustible cartridge casing.

10. The process according to claim 1 in which the material contains from about 15 to 35% of a cured resin binder derived from a self-reactive copolymer containing at least by Weight, of vinyl acetate together with at least one other polymerizable comonomer containing at least one functional group selected from the group consisting of the glycidyl, methylol, ureido, hydroxyl, amine, carboxyl and alkoxycarbonyl groups.

11. A combustible shaped, felted, fibrous material comprising a minor amount of synthetic organic resin binder and a major amount of nitrocellulose fibers subjected to a solvating treatment in accordance with the process of claim 1.

12. A combustible shaped, felted, fibrous material comprising a minor amount of synthetic organic resin binder and a major amount of nitrocellulose fibers subjected to a solvating treatment in accordance with the process of claim 10.

13. The combustible material of claim 11 in the form of a cartridge casing.

14. The combustible material of claim 12 in the form of a cartridge casing.

References Cited by the Examiner UNITED STATES PATENTS 2,982,211 5/1961 Beal et al. 102-43 3,139,355 6/1964 De Fries et al. 14996 X 3,218,907 11/1965 Beal et al. 86-10 3,236,704 2/1966 Axelrod et a1. 149-49 BENJAMIN R. PADGETT, Primary Examiner. 

1. THE PROCESS WHICH COMPRISES SUBJECTING A COMBUSTIBLE, SHAPED, FELTED, AND FIBROUS MATERIAL CONSISTING ESSENTIALLY OF NITROCELLULOSE FIBERS, CELLULOSE FIBERS AND SYNTHETIC ORGANIC RESIN BINDER THEREFOR TO A SOLVATING TREATMENT WITH SOLVENT FOR NITROCELLULOSE FOR A PERIOD OF TIME SUFFICIENT TO SOLUBILIZE AT LEAST THE OUTER SURFACES OF SAID NITROCELLULOSE FIBERS, AND THEN REMOVING THE SOLVENT.
 10. THE PROCESS ACCORDING TO CLAIM 1 IN WHICH THE MATERIAL CONTAINS FROM ABOUT 15 TO 35% OF A CURED RESIN BINDER DERIVED FROM A SELF-REACTIVE COPOLYMER CONTAINING AT LEAST 90%, BY WEIGHT, OF VINYL ACETATE TOGETHER WITH AT LEAST ONE OTHER POLYMERIZABLE COMONOMER CONTAINING AT LEAST ONE FUNCTIONAL GROUP SELECTED FROM THE GROUP CONSISTING OF THE GLYCIDYL, METHYLOL, UREIDO, HYDROXLY, AMINE, CARBOXYL AND ALKOXYCARBONYL GROUPS. 